2.3. Photometer

After the intermediate focus provided by the entrance optics,
the light is split into the long-wavelength and short-wavelength channels
by a dichroic beam-splitter with a transition wavelength of 125 µm
and is re-imaged with different magnification onto the respective Si bolometer arrays.

The blue channel offering two filters, 60-85 µm and 85-125 µm,
has a 32x64 pixels arrays, while the red channel with a 125-210 µm filter
has a 16x32 pixels array.
Both channels cover a field-of-view of ~1.75'x3.5', with full beam sampling in each band.
The two short-wavelength bands are selected by two filters via a filter wheel.
The field-of-view is nearly filled by the square pixels,
however the arrays are made of sub-arrays which have a gap of ~1 pixel in-between.

The incident infrared radiation is registered by each bolometer pixel by causing
a tiny temperature difference.

2.3.1. Filters

The PACS filters, in combination with the detectors, define the photometric bandpass of the
instrument.
There are in total 3 bands in the PACS photometer: 60-85 µm, 85-125 µm
and 125-210 µm.
The PACS filter scheme is shown in
Figure 2.4
and the filter transmission of the photometer filters in
Figure 3.5.

Figure 2.4. Overview of the filter arrangements in PACS.
The selection of the blue photometer filter is done via commanding of the filter wheel 2.

2.3.2. Bolometer arrays

Figure 2.5 shows a cut-out of the 64x32 pixel
bolometer array assembly.
4x2 monolithic matrices of 16x16 pixels are tiled together to form the short-wave
focal plane array.

Figure 2.5. Bolometer matrices assembly: 4x2 matrices from the focal plane of the short-wave bolometer assembly.
The 0.3 K multiplexers are bonded to the back of the sub-arrays.
Ribbon cables lead to the 3K buffer electronics.

In a similar way, 2 matrices of 16x16 pixels, are tiled together for the long-wavelength focal plane array.

The matrices are mounted on a 0.3K carrier which is thermally isolated from the surrounding 2K structure.
The buffer/multiplexer electronics are split in two levels; a first stage is part of the indium-bump bonded
back plane of the focal plane arrays, operating at 0.3K.
Ribbon cables connect the output of the 0.3K readout to a buffer stage running at 2K.

For science observations the multiplexing readout samples each pixel at a rate of 40 Hz
Because of the large number of pixels, data compression by the SPU is required.
The raw data are therefore binned to an effective 10 Hz sampling rate.
After that, the same lossless compression algorithm is applied as with the spectrometer data.

2.3.3. Cooler

The photometer operates at sub-Kelvin temperatures, which are achieved using a
3He cooler.
This type of refrigerator uses porous material which absorbs or releases gas depending
on the mode: cooling or heating. The use of the 3He isotope
instead of the common 4He is dictated by two reasons:
it is not super fluid at cryogenic temperatures below 2.2 K and it is a superior cryogen.
This sorption cooler is run from a cold stage provided by the Herschel cryostat.
The refrigerator contains 6 litres of 3He and can in principle
be recycled infinitely, with an efficiency of more than 95% with a lifetime
limited only by the cold stage from which it is run.
Gas-gap heat switches, which are coupled to the Herschel 3K system with thermal straps,
control the mode of operations.
The evaporation of 3He provides a very stable thermal
environment under constant heat load.
The design of the cooler is well suited
for work in space as there are no moving parts and the heat load is small.

This sorption cooler is nearly identical to the unit developed for SPIRE.
It provides a stable temperature environment at 300 mK for more than 48 hours
under normal observing and operational circumstances.
The recycling is performed during DTCP periods, when the PACS photometer
is selected for the following observing day and has a hold time of ~60h,
allowing of up to 2.5 consecutive ODs of operations.